A known apparatus is provided, by way of example, by a mobile communications network in which data packets are transmitted between a mobile terminal and a node in the mobile communications network.
The transmitted data packets include a control data part and a user data part. The structure of such a data packet is described below using the example of transmission of the data packet from a mobile terminal (UE) to a node in the mobile communications network (radio network controller, RNC): in the example, it is assumed that the UMTS protocol architecture is present both in the mobile terminal (UE) and in the node in the mobile communications network (RNC). The UMTS protocol architecture for layers 2 and 3, which are relevant in this case, is described, by way of example, in the document “3G TS 25.301, UMTS Protocol Architecture, 3 GPP, March 2000”.
The mobile terminal can generate data for various applications. For transmitting voice, a voice code, for example, generates one or more voice data streams or an HTML browser generates non-uniform packet data streams. These data, which are user data, are first possibly modified by protocols for higher layers and are prepared for data transmission in the mobile communications network. In this connection, the User Datagram Protocol (UDP) and the Internet Protocol (IP) can be used. For transmission via the UMTS radio interface, these user data need to be optimized in the various protocols for layer 2 of the UMTS protocol architecture.
It is also possible for non-UMTS-specific protocols to use the UMTS radio interface. The associated service access point within the UMTS protocol architecture is called the radio bearer (RB). The RB service access point is therefore provided above layer 2 and transmits user data transparently from the mobile terminal, via the UMTS radio interface, to the node in the mobile communications network, and vice versa. For data transmission, a particular transmission quality of service (QoS) is stipulated when such an RB service access point is set up, said transmission quality of service being distinguished, by way of example, by a particular, guaranteed data rate, a maximum transmission delay and/or a maximum bit error rate.
When preparing and optimizing the user data, for example voice data streams, various packet-switching-specific protocols, such as IP, TCP, UDP, etc., are executed, the user data part being preceded by control data which are required for routing and for “session setup” in IP-based networks. These control data are not needed for transmitting a data packet via the radio interface of the mobile communications network. For efficient transmission of a data packet, the control data are therefore compressed. This is done by executing suitable algorithms in the PDCP layer of the UMTS protocol architecture, in that case layer 2.
The data packets with a compressed control data part are transferred to the RLC layer of the UMTS protocol architecture for further processing. The RLC layer is used when the length of the data packets coming from the PDCP layer, which data packets can be of any length, needs to be optimized for the radio interface. A further function of the RLC layer is the performance of error correction. If it is necessary neither to optimize the length of the data packets nor to correct errors, the RLC layer can be operated in “transparent mode”. Each RB service access point has an associated RLC layer which can also be used for buffer-storing the data present at an RB service access point.
Arranged in relation to the UMTS protocol architecture below the RLC layer is the MAC layer, whose service access points for the RLC layer are called logical channels. The task of the MAC layer is essentially to distribute the data packets received from the RLC layer on the various channels dynamically over the available transport channels. Transport channel refers to a service access point in the physical layer for the MAC layer.
Normally, for each transport channel, permanent coding is set which governs the transmission error rate for data packets and also the necessary bandwidth for transmission. By way of example, convolution coding with the rate 1/3 can be set for a transport channel, so that the data rate transmitted via the radio interface is three times as high as the data rate transmitted from the RLC layer to the MAC layer. The transmission error rate for transmission using such coding is much lower than when convolution coding with the rate 1/2 (doubling of the data rate) or even no coding is used.
Conventionally, an individual transport channel is available for transmitting data packets from a logical channel, which means that a transport channel and hence the transport channel's fixed data coding is defined for every logical channel. However, it is possible for a plurality of logical channels to use the same transport channel.
On the basis of the prior art, the data packets are thus transmitted in full via a particular transport channel for which the coding has been permanently set. This coding increases the volume of data to be transmitted per data packet, specifically to a greater extent the better the error elimination rate and hence the lower the residual error rate after the decoding which is carried out in the reception device—in the present example, at the node in the mobile communications network.
Following transmission, the control data parts of the data packets are removed in order to recover the original user data, such as voice data streams.
The aforementioned compression of the user data part of a data packet in the communication apparatus' transmission device for more efficient transmission of the data packet via the radio interface requires a high transmission quality of service, since incorrect transmissions can spread to a multiplicity of subsequent data packets. Since user data and control data are transmitted using the same coding, the transmission of a data packet is found to require a considerable bandwidth to be available in a transport channel.